Histamine is an important biogenic amine which functions in diverse physiologic roles in mammals. For example, histamine plays a role in allergic hypersensitivity reactions, modulation of gastric acid secretion, and is believed to function as a neurotransmitter in the central nervous system. For a detailed discussion of histamine and its functions within the human body, see chapter 26 of Goodman and Gilman's The Pharmacological Basis of Therapeutics, 6th Edition 1980.
Due to the importance of histamine in biologic systems, accurate and reliable methods for determining the level of histamine in the body are critical to provide proper diagnosis. Further, since histamine is present in the body in very small amounts, the methods must be highly sensitive, that is, capable of detecting the compound in very small amounts. The frequency at which these methods are conducted mandate further that they be highly reproducible under laboratory conditions when employing a variety of body tissues and fluids and provide the results quickly in order to facilitate patient recovery.
Radioenzymatic assays are sensitive analytical methods which have found wide use in the quantification of various biogenic amines, including histamine. These methods are based on the enzyme catalyzed methylation of a substrate of interest to a radiolabeled product using radiolabeled S-adenosylmethionine as the methyl donor. The radiolabeled reaction product is then selectively isolated and quantified by scintillation spectrometry.
The histamine N-methyltransferase catalyzed methylation of histamine to tritiated N-.tau.-methylhistamine has been widely used as the basis for development of a histamine radioenzymatic assay. However, due to the use of relatively impure enzyme preparations, current procedures lack the necessary sensitivity and specificity to quantify histamine in important biologic samples, such as human plasma and urine.
Accordingly, the present invention describes an improved radioenzymatic assay for histamine employing highly purified histamine N-methyltransferase. The use of purified histamine N-methyltransferase in the present histamine radioenzymatic assay dramatically improves assay sensitivity and specificity by removing competing methyltransferases and small molecules. Therefore, the use of purified histamine N-methyltransferase permits the development of a substantially improved assay system for the quantification of histamine in biologic samples.